An air gap between the patient and the film-screen will improve contrast of the film by reducing the obliquely scattered rays. There will be a large reduction in the intensity of the scattered radiation; a small reduction in the intensity of the primary radiation will also result due to the inverse square law. Therefore, an increase in the kV or mAs is required with an air gap because the subject is nearest to the x-ray source.
A common use of the air gap is in magnification mammography. Since an air gap is produced by separating the breast from the receptor to produce magnification, it can be used for scatter reduction. Spatial resolution required to study the microcalcifications in magnification mammography is obtained with the reduction of the focal spot. The exposure time gets longer because the microfocus has lower mA values and lower power than the standard fine focus; in addition, the exposure time must be increased to compensate the greater distance between the breast and the image receptor. Because the breast is nearest to the x-ray source, the amount of given radiation would be very higher. The usual procedure is to remove the grid and rely on the air gap.
- 1. Allisy-Roberts P, Williams J. Farr's Physics for Medical Imaging. W.B. Saunders Company. (2007) ISBN:0702028444. Read it at Google Books - Find it at Amazon
- 2. RSNA, Syllabus: A comprehensive course in physics technical aspects of breast imaging, 1982
- 3.G.F.PISTOLESI e M.TONEGUTTI : “Presupposti tecnico-biologici all'immagine. mammografica” 138-160. Cortina Editrice., Verona, 1987
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